This invention relates generally to ultrasonic systems for measuring the flow of liquid through a pipe, and in particular to a system which includes means to determine the geometry of the flow pipe and to factor-in the geometry in the flow rate measurement.
The use of ultrasonic techniques to determine the flor rate of a liquid flowing through a pipe is well known. Among patents disclosing systems of this type are U.S. Pat. Nos. 4,103,551; 4,004,461 and 3,906,791. Typically, in an ultrasonic system adapted to measure flow rate, a pulse of ultrasonic energy is alternately emitted by a pair of transducers; one transducer being placed upstream on the pipe at one end of a tilted diameter or diagonal, the other being placed downstream at the other end of the diagonal. The direction of tilt lies in the direction of flow.
A pulse of ultrasonic energy emitted by one transducer and propagated through the fluid being metered at an angle to the longitudinal flow axis is detected by the other transducer. The time delay difference (that is, the upstream time minus the downstream time) between the generation of the emitted pulses and their reception is a function of flow velocity, and therefore may be converted into a flow rate reading.
In order to maintain the required non-normal angle between the longitudinal axis of the fluid flowing in the pipe and the sonic velocity vector, it is often the practice to mount each ultrasonic transducer within a pocket protruding into the pipe. Such protrusions are objectionable, for they disturb the local flow velocity profile in the region in which measurements are to be made, and also create a collection point for solids dispersed in the liquid. Other ultrasonic flowmeters employ oblique or wedge injection techniques to obviate the need for pockets. Thus in the U.S. Pat. No. 4,195,517 to Kalonoski et al., each crystal transducer is coupled to the pipe by means of a wedge-shaped sonic probe of stainless steel or other solid material capable of transmitting acoustic pulses.
In the Kalinoski et al. U.S. Pat. No. 4,195,517, whose entire disclosure is incorporated herein by reference, a pair of transducers is mounted externally on the same side of the pipe, each transducer, in turn, being physically coupled to a sonic probe which transmits and receives generated acoustic pulses between the transducers through the flowing fluid. The transducers and probes are arranged so that the acoustic pulses enter the pipe at one side thereof at an oblique angle to the solid-fluid interface, at which point refraction occurs. The pulses then propagate through the fluid to the opposite wall of the pipe where they are reflected toward the other transducer.
In the Kalinoski et al. system, the acoustic path through the fluid is determined for a given set of conditions by the ratio of the velocity of sound in the probe and the velocity of sound in the fluid in accordance with Snell's Law. The transducers are excited sequentially to produce acoustic pulses which first travel from the upstream to the downstream transducer and then in the reverse direction. A gated counter measures the respective sonic propagation times between the two transducers. The functions of exciting the transducers, measuring the sonic propagation times, reading the values of process and geometric parameters, and making the necessary calculations are carried out by digital techniques employing a microprocessor.
The geometry of the flow pipe is a major factor in ultrasonic volumetric flow measurement. Ultrasonic meters of the clamp-on type usually assume a nominal geometry; and if this assumption is correct, the flow rate measurements are accurate to the extent that the system is capable of providing a reading that is linearly proportional to flow rate. But since in actual practice, pipe tolerances vary from installation to installation, this uncertainty may give rise to gross errors, particularly in those installations where corrosion and scaling of the pipe has occurred.